1. Field of the Invention
This invention relates to a blend of compounded natural rubber and staple fibers. More particularly, the invention concerns such a blend in which the staple fibers are spandex.
2. Description of the Prior Art
It is known to blend fibers into rubber compositions. Typically, fibers of high-strength and high-modulus, such as fibers of nylon, polyester, glass, steel, or aramid fibers, are used in the blends. The blending of other types of fibers in various rubber compositions also is known.
Japanese Patent Application Publication No. 4-13792 discloses elastomeric sheet compositions containing 1 to 15 weight percent of plastic fibers of ultra-fine denier (i.e., 0.001 to 0.9 denier) mixed in a rubber binder composition. Numerous kinds of plastic fibers, as well as numerous kinds of synthetic rubbers and natural rubber, are listed as being suited for use in the composition. The ultra-fine plastic fibers are derived from composite fibers which comprise two or more polymers, preferably nylon and polyester. The composite fibers are splittable into fibers of very fine-denier. Among the many polymers listed as being suited for one of the polymers in the composite bicomponent fibers polyurethane is included.
Japanese Patent Application Publication No. 5-63508 discloses the mixing of polyurethane fibers (spandex) in a liquid silicone rubber matrix, heating the mixture and then pressing the mixture into sheets. A special treatment is employed to adhere the fibers to the silicone rubber.
Japanese Patent Application Publication No. 62-112618 discloses mixing 0.5-cm-long, 40-denier spandex into a liquid polybutadiene diol, reacting the mixture with diphenymethane diisocyanate to form a polyurethane elastomer matrix containing the spandex, and then casting a tire therefrom.
The present inventor found that significant improvements are made in the tensile strength, tear resistance, and elongation at break of a vulcanized sheet of natural rubber, if polyurethane staple fibers (i.e., spandex) are blended into the natural rubber.